Servovalves find a wide range of applications for controlling air or other fluid flow to effect driving or control of another part e.g. an actuator.
A servovalve assembly includes a motor controlled by a control current which controls flow to an air valve to control an actuator. Generally, a servovalve transforms an input control signal into movement of an actuator cylinder. The actuator controls e.g. an air valve. In other words, a servovalve acts as a controller, which commands the actuator, which changes the position of an air valve's (e.g. a so-called butterfly valve's) flow modulating feature.
Such mechanisms are used, for example, in various parts of aircraft where the management of air/fluid flow is required, such as in engine bleeding systems, anti-ice systems, air conditioning systems and cabin pressure systems. Servovalves are widely used to control the flow and pressure of pneumatic and hydraulic fluids to an actuator, and in applications where accurate position or flow rate control is required.
Conventionally, servovalve systems operate by obtaining pressurised fluid from a high pressure source which is transmitted through a load from which the fluid is output as a control fluid. Various types of servovalves are known—see e.g. GB 2104249, US 2015/0047729 and U.S. Pat. No. 9,309,900.
Electrohydraulic servovalves can have a first stage with a motor, e.g. an electrical or electromagnetic force motor or torque motor, controlling flow of a hydraulic fluid to drive a valve member e.g. a spool valve of a second stage, which, in turn, can control flow of hydraulic fluid to an actuator for driving a load. The motor can operate to position a moveable member, such as a flapper, in response to an input drive signal or control current, to drive the second stage valve member e.g. a spool valve.
Particularly in aircraft applications, but also in other applications, servovalves are often required to operate at various pressures and temperatures. For e.g. fast acting air valve actuators, relatively large flows are required depending on the size of the actuator and the valve slew rate. For such high flow rates, however, large valve orifice areas are required. For ‘flapper’ type servovalves, problems arise when dealing with large flows due to the fact that flow force acts in the direction of the flapper movement and the motor is forced to overcome the flow forces. For clevis-like metering valves such as described in U.S. Pat. Nos. 4,046,061 and 6,786,238, the flow forces, proportional to the flow, act simultaneously in opposite directions so that the clevis is balanced and centered. The clevis, however, needs to be big due to the requirement for bigger orifices to handle larger flows.
Jet pipe servovalves provide an alternative to ‘flapper’—type servovalves. Jet pipe servovalves are usually larger than flapper type servovalves but are less sensitive to contamination. In jet pipe systems, fluid is provided via a jet pipe to a nozzle which directs a stream of fluid at a receiver. When the nozzle is centered—i.e. no current from the motor causes it to turn, the receiver is hit by the stream of fluid from the nozzle at the centre so that the fluid is directed to both ends of the spool equally. If the motor causes the nozzle to turn, the stream of fluid from the nozzle impinges more on one side of the receiver and thus on one side of the spool more than the other causing the spool to shift. The spool shifts until the spring force of a feedback spring produces a torque equal to the motor torque. At this point, the nozzle is centred again, pressure is equal on both sides of the receiver and the spool is held in the centered position. A change in motor current moves the spool to a new position corresponding to the applied current.
As mentioned above, jet pipe servovalves are advantageous in that they are less sensitive to contamination e.g. in the supply fluid or from the valve environment. These valves are, however, more complex and bulkier. The torque motor that comprises electromagnets to apply electromagnetic force to an armature to move the jet pipe is large and heavy, which adds to the size, weight and complexity of the overall system.
There is a need for a servovalve arrangement that can handle large fluid flows effectively, whilst retaining a light, simple and compact design.
The present disclosure provides a servovalve comprising: a fluid transfer valve assembly comprising a supply port and a control port; a moveable valve spool arranged to regulate flow of fluid from the supply port to the control port in response to a control signal; and a drive assembly configured to axially move the valve spool relative to the fluid transfer assembly in response to the control signal to regulate the fluid flow; wherein the drive assembly comprises a steerable member moveable by an amount determined by the control signal to cause corresponding movement of the valve spool; the drive assembly further comprising piezoelectric actuator means configured to move said steerable member in response to the control signal.
The fluid transfer valve assembly may also comprise a return port for pressure returning through the assembly.
The steerable member may comprise a flapper-type member or a jet-pipe with nozzle which can be steered in response to the control signal. The steerable member may be mounted within an armature and/or a flexible pipe.
In a jet-pipe system, a nozzle is preferably provided at an end of a jet pipe closest to the valve assembly and fluid from the nozzle is directed into the valve assembly via a receiver. The receiver is preferably configured such that when the nozzle is in a central position, fluid enters the valve assembly evenly via both sides of the receiver, e.g. by opposing lateral receiver channels. When the nozzle is steered to an off-centre position, more fluid flows to one side of the valve assembly than the other via the receiver; e.g. more flows through one lateral receiver channel than the other.
The piezoelectric actuator means preferably comprises a plurality of piezoelectric elements arranged to impart movement to the steerable member. In one embodiment, the piezoelectric actuator means comprises two piezoelectric elements mounted on opposite sides of the steerable member.
Preferred embodiments will now be described with reference to the drawings.